Harmful algal traits and bloom dynamics under climate change

Anthropogenic activities have caused a rapid increase in greenhouse gas emissions, such as CO2, which also led to a rise in global mean temperatures. Next to changes in climate conditions, anthropogenic perturbations also include the release of vast amounts of nutrients into coastal waters. These global alterations in environmental conditions have severe consequences for marine phytoplankton species, which are responsible for half of all primary production on Earth. Some phytoplankton species, however, can become a nuisance for the environment through the formation of dense harmful algal blooms (HABs). Especially HABs caused by dinoflagellates can have far-reaching consequences, as they are known to produce potent toxins that may be detrimental for the environmental and human health. The aims of this thesis were to assess the environmental drivers behind HAB development, determine the extent of intraspecific trait variation in HAB populations, and to evaluate the impact of climate change on HAB proliferations. Although nutrient dynamics play a substantial role in the emergence of HABs, other biotic and abiotic factors may strongly modulate the magnitude and duration of bloom events. For instance, low salinities due to excessive rainfall and increased wind speeds led to significant reductions in Alexandrium ostenfeldii bloom densities in the Netherlands, while highest population densities generally corresponded to high temperatures, low N:P ratios and low grazer densities. This demonstrates the important role of the combination of physical, chemical and biological factors in the development of HABs. An important factor that may contribute to the success of HAB species is the considerable intraspecific trait variation that can be found within populations. Strains derived from two A. ostenfeldii populations expressed substantial phenotypic variation in functional traits, such as growth rate, cell size, toxin production, allelopathic potency, elemental stoichiometry, and nitrogen uptake kinetics. This observed high trait variation may facilitate development and resilience of HABs, especially under changing environmental conditions. Different environmental variables may also influence phenotypic trait expression. For instance, elevated CO2 concentrations caused an increase in growth rates in three A. ostenfeldii strains derived from the same population. Phenotypic plasticity in trait responses, and variation therein, towards environmental stressors may be important for species adaptation, especially on the short term. Although substantial variation in trait responses of HAB species towards climate change variables can be found, I did identify clear trends when combining data from a multitude of culture experiments. Specifically, HAB growth rates showed an overall increase in response to elevated pCO2. This may represent a competitive advantage for HAB species in future waters, particularly since a similar trend was not found for other phytoplankton species. In addition, elevated temperatures also led to an increase in growth rates, but only for HAB species isolated at higher latitudes. Since the success of HAB species ultimately depends on growth rates, these findings warn for a greater potential of HAB development in future oceans, particularly in temperate regions. Overall, the results of this thesis contribute to a better understanding of dinoflagellate HAB dynamics and the potential impacts of climate change on HAB proliferations